Mobile telecommunications devices continue to be manufactured in smaller and smaller packages. As a result, the electronic components within mobile telecommunication devices must also continue to be manufactured in smaller and smaller packages. A typical mobile telecommunication device uses a receiver and a transmitter to receive and transmit communication signals with other telecommunications devices. In receiving communication signals, a typical telecommunications device uses one or more filters to isolate the communication signal desired as there may be additional signals resident within a larger signal, e.g., a carrier signal, or a noise signal. As such, many different types of filters have been developed for the purpose of isolating particular portions of communication signals received by a receiver.
One type of filter often used in conjunction with well-known receiver architectures is a lattice filter. FIG. 1A is a schematic diagram of a conventional single-stage lattice filter 100 attached to a conventional balun 105 The lattice filter 100 is shown attached to the balun 105 because filters utilizing resonators typically have a balanced input and a balanced output. At the same time, typically these filters are connected to a conventional antenna which is an unbalanced, i.e., a single transmission path. but it is difficult to implement such a configuration with resonators. Therefore, the lattice filter 100 uses the balun 105 which is a signal line transformer for converting an unbalanced signal to a balanced signal. The balun 105 includes an unbalanced side 106 connected directly to the unbalanced input 101 and a balanced side connected to the balanced input 107 of the filter 100.
The lattice filter 100 includes a balanced input 107 and a balanced output 102. The filter 100 also includes four resonators 110a–d connected between the balanced input 108 and the balanced output 102 in the schematic arrangement shown in FIG. 1A.
The resonators 110a–d may be bulk acoustic-wave (BAW) devices that are, in general, comprised of a piezoelectric layer disposed between two electronically conductive layers that serve as electrodes. As such, when a radio frequency (RF) signal is applied across a resonator, a mechanical wave is generated in the piezoelectric layer. A fundamental resonance occurs when the wavelength of the mechanical wave is about twice the thickness of the piezoelectric layer. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonance frequency is increased. BAW devices have traditionally been fabricated on sheets of quartz crystals. In general, it is difficult to achieve a device of high resonance frequency using this fabrication method. When fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonance frequency to the 0.5–10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. FBAR resonators and the nature of their manufacturing are well known in the industry and are not discussed further herein.
Thus, when disposed on the substrate, each resonator 110a–d includes a top electrode 111a–d and a bottom electrode 112a–d that remain electrically isolated from one another by the piezoelectric material. By coupling the resonators in the schematic manner shown in FIG. 1A, an RF signal received at the unbalanced input (which is typically connected to a radio antenna (not shown)) can be filtered by adjusting the resonance frequencies and relative sizes of the resonators 110a and 110c in relation to those of resonators 110b and 110d. The physical wave characteristics and electronic aspects of lattice filters are also well-known in the industry and are not discussed further herein.
The balanced input 107 of the filter 100 comprises a first input terminal 108a and a second input terminal 108b. The first input terminal 108a is connected to the top electrodes 111a and 111b of the resonators 110a and 110b. Likewise, the second input terminal 108b is connected to the top electrodes 111c and 111d of the resonators 110c and 110d. In a similar manner, the balanced output 102 includes a first output terminal 103a and a second output terminal 103b. The first output terminal 103a is connected to the bottom electrodes 112a and 112d of the resonators 110a and 110d. Likewise, the second output terminal 103b is connected to the bottom electrodes 112b and 112c of the resonators 110b and 110c. 
In this arrangement, a crossover 120 is necessary because of the cross-connected nature of the lattice arrangement. This is most typically accomplished by providing vias through the substrate in order to utilize both the top and bottom sides of the substrate to provide a signal path at the crossover 120. This is illustrated more clearly in the physical topology of the lattice filter 100 in FIG. 1B.
FIG. 1B is a top view a portion of the lattice filter 100 of FIG. 1A showing the physical topology of the crossover 120 between the resonators 110a–d. In general, the shaded areas of FIG. 1B represent the bottom electrodes of the illustrated resonators and the non-shaded areas represent the top electrodes of the illustrated resonators. As can be seen, the balanced input 108 includes the first input terminal 108a and the second input terminal 108b which are connected to the top side electrodes, i.e., the first input terminal 108a is connected to top electrodes 111a and 111b of the resonators 110a and 110b and the second input terminal 108b is connected to top electrodes 111c and 111d of the resonators 110c and 100d. Likewise, the balanced output 102 also includes the first output terminal 103a and the second output terminal 103b which are connected to the bottom electrodes, i.e., the first output terminal 103a is connected to the bottom electrodes 112a and 112d of the resonators 11a and 110d and the second output terminal 103b is connected to the bottom electrodes 112b and 112c of the resonators 110b and 110c. 
The crossover 120 is realized by connecting the bottom electrodes of the diagonal resonators together, i.e., the bottom electrode 112b of resonator 110b to the bottom electrode 112c of resonator 10c and the bottom electrode 112d of resonator 110d to the bottom electrode 112a of resonator 110a. However, because the signal path must cross due to the arrangement of the resonators 110a–d, one of the two afore-mentioned connections must “crossover” the other. This is accomplished by etching the piezoelectric layer (not shown in detail) with vias that allow an electrical connection through the piezoelectric material. In particular, two vias 121a and 121d (not shown in detail) are disposed at two points which are near each respective bottom electrode 112a and 112d of the connecting resonators 110a and 110d. Using the vias 121a and 121d, a crossbar connector 122 disposed on the top side of the piezoelectric layer provides a connection between the vias 121a and 121d, i.e., an electrical connection between the bottom electrodes 112a and 112d. As such, electrical signals are able to travel from the bottom electrode 112a of the resonator 110a, to the first via 121a, through the via 121a to the crossbar connector 122 to the second via 121d, through the second via 121d to the bottom electrode 112d of the resonator 110d without ever interfering with the signal path between the bottom electrodes 112b and 112c of the resonators 110b and 110c which remains disposed entirely on the bottom side of the piezoelectric layer.
There are several drawbacks to this crossover arrangement of resonators 110a–d in a lattice filter 100. First, vias 121a and 121d require a specific amount of space to realize in the piezoelectric layer. As such, space is wasted between each resonator 110a–d because enough space must be left between each of the resonators 110a–d in order to achieve the crossover 120 using vias 121a and 121d. Since electronic components in mobile telecommunications devices are becoming increasingly smaller, the space between resonators 110a–d in lattice filters could be utilized in a more efficient manner.
Second, as signal path traces become smaller, electrical losses to signals propagating through the connection lines increase. Thus, if the signal path traces at the crossover 120 were to be designed smaller, the electrical losses to signals increase. Thus, wider signal path traces will prevent higher electrical losses, but at the expense of space being increased between the resonators 110a–d. Thus, the signal path traces can only become smaller to a point in which the electrical losses to the RF signal render the RF signal unrecognizable.
Third, even if space is not a limiting factor such that the signal path traces in the crossover 120 may be as wide as necessary so as to minimize electrical losses, the crossover point still creates a large “dead” resonator between the top and bottom side signal path traces. This dead resonator generates an appreciable acoustical loss in the RF signal. If the dead resonator becomes large enough, i.e., the signal path traces at the crossover point are large enough, the acoustical losses in the RF signal again will degrade the filter performance.
Thus, in a lattice filter arrangement utilizing a crossover connection, a constant tradeoff is negotiated in designing a filter that balances the acoustical losses to the RF signal if designed to be larger and the electrical losses to the RF signal if the designed to be smaller. The crossover poses a problem of not allowing the filter to be disposed in as little space as possible because there must remain a finite amount of space between resonators in order to dispose the vias between the resonators so as to couple the resonators in the correct filter schematic arrangement.